fluctuating speedometer needle and cruise control
 

The speedometer needle on my 76 300D (115 chassis) fluctuates intermittently by up to 5 mph between 25 and 80 mph, and it is doing it more often lately. The greatest fluctuation is when the car is slowing down (the needle bobs up and down by 10 mph or more between 25 and 10 mph).

When the needle is fluctuating and I engage the cruise control (pneumatic CC actuator), the car "pulses"...I may eventually develop whiplash!

When the needle is not fluctuating, the CC (new amp) is also steady (the speed sensor is in-line and the 2 halves of the speedometer cable are joined at the sensor).

Is the fluctuation more likely the result of the cable or the speedometer - or both? Or is the speed sensor the likely culprit?

If it is the cable, what is involved in replacing the lower half of the cable (from trans to in-line speedsensor) - unfortunately there is almost nothing in the paper and CD manuals I have about the transmission).

Robert

My bet is on the cable or its routing. It should make no narrow radius bends. The cables are cheap. I would replace it.

Steve,

Thanks for the reply.

There are no noticeable bends in the cable...the car is near mint and has about 128K on it...the 2 cables are about $125.

What is involved in relacing them...the cable from the sensor to the speedometer is easy enough, but from the trans to the sensor I'm not sure.

MB treats transmission information like it were a national secret, and I haven't been able to get a trans manual.

How (and where) is the cable attached to the trans, and what "secrets" do I need to know to replace it? I have the tools and the time but not the know how.

Robert

Disconnect the cable at the sensor and shoot some lube down both sections especially the lower. Use the snorkle. I did this to my truck and the fluctuation and sqweek went away. Be sure to put plenty in the lower as it will work its way down.

What lube(s) would you suggest...graphite, silicone, ATF, light machine oil, ??? What is the best and most likely to take care of the problem for more than a short period of time?

I used lithium grease. It comes out of the spray can liquid and then thickens sometime later as it is exposed to the air. I got another 3 years on that cable doing this. Also I replaced the lower cable on my76 300d. The ends of the cables are square. At the transmission there was only one bolt on a collar that holds the cable in. You might remove it entirely hang it up and let gravity help to get the lube down the entire length. They used to sell a cable injector at motorcycle shops $10. But that was 20 yrs ago.

The speedo cable on a MB can not be separated from the housing making usefull lubrication a hit or miss (mostly miss) proposition.

Steve,

When you replaced the lower cable/housing, how much clearance at the trans was there to work with...the last t ime I was underneath the car it seemed there was very little room to work where the cable connects to the trans (toward the rear of the trans).

What was your experience...tight fit or was I looking at the wrong line?

Robert

You have to keep in mind my car was made in 1976. They are simpler than the later models. I had plenty of clearance and the only tough part was seating the feed through rubber. I Lubed my cable prior to installation. Being of 70's technology. I don't remember what year you have but it may be of a different design.

Steve,

I also have a W115 '76 300D (super car!!).

I am a bit confused (what do you mean by "seating the feed through rubber")...did you replace the speedo cable itself (feed the square- shaped metal cable through the housing ) or did you feed the housing/cable combination through some other piece of rubber?...the moderator (Stevebfl) suggests that the cable and housing are inseparable.

Any installation details that you can remember would be greatly appreciated...I've replaced speedometers on older American cars but this looks a bit different...am I overreacting?

Robert

The cables should be straight forward, to install. The end on the trannie is released by simply loosening and removing the pinch bolt (10mm wrench size). Pull out the bolt and pull out the cable. The cable will have a grommet made on it to seal the firewall hole that it goes through. Getting this grommet to its proper installed position is a little tricky, but the rest is simple (If the car is on a lift for the bottom part)

I should have called it a rubber grommet. It is the weather seal from underneathe to the passenger compartment. It is a tight fit, some vasalline on the small side may help seat it easier.

Robert,

I don't know if you have completed the job yet, but for what it's worth ...

I had exactly the same problem on my '77 450SEL (116 chassis). Replacing the speedometer cable fixed it completely. My cable was housing and cable. And if they are available separately, I would replace them both anyway. Both can become worn and I would imagine they both have after the miles you must have by now.

I would be reluctant to shoot new lube in the cable before installation. The factory has put proper lubrication in there and second-guessing them may just as likely wash out the proper lube.

It's been a couple years now, so I don't remember any details, but I do remember it was straightforward and much easier than I anticipated. As I recall I didn't know what to do with the rest of my Saturday afternoon* I'm sure I found something else on the car to fix**

I also recall that when talking to the speedometer mechanic, he mentioned that wear on the bushing the speedometer needle shaft fits through can also cause jumping. This is the cause I suspect on the same behavior on my '88 Volkswagen 136k mi. On it I just replaced the cable and although jumping is much less, the speed fluctuates to 10-20 mph higher than actual at highway speeds. To fix this you would need to remove the speedometer and send it to or leave it with a speedometer repair shop for the modest repair.

Good luck with your repairs*

Cheers,
sc